1. Field of the Invention
The present invention concerns a circuit for compensating the errors occurring when changing the playing back speed of a double azimuth 4-head VTR, and more particularly means for compensating vertical and horizontal skew errors during the playing back speed change to improve the picture quality.
2. Description of the Related Art
Conventionally, as shown in FIG. 1, the signals of the double azimuth four heads HSP1, HSP2, HEP1 and HEP2 are connected via amplifiers SA1, SA2, EA1 and EA2 to switches S1 and S2 switched by switching pulse signals HSW, and the signals a and b loaded on the common terminal of said switches S1 and S2 are applied to a comparator to generate head comparing switching signals. In addition, the signals loaded on the common terminal of the switches S1 and S2 are connected to a control switch S3 controlled by a multiplexer 20 for receiving the head comparing switching signals and head switching pulses SP/EP to transmit data, thus producing high frequency signals of a pre-amplifier. In such conventional circuit comprising the double azimuth for heads HSP1, HSP2, HEP1 and HEP2, the spatial interval between two heads is greater in the specified long time speed heads HEP1 and HEP2 than in the standard speed heads HSP1 and HSP2 in the drum rotating speed, thus resulting in a double azimuth head with about 2H (H is TV scanning lines) time interval.
The spatial interval between a pair of the double azimuth heads is 2H.+-..alpha. in the drum rotating speed, as shown in FIG. 2. Hence, when changing the speed, head output waveforms as shown in FIGS. 4(a) and 4(b) and the high frequency output waveform as shown in FIG. 4(c) appear so that parts of the picture are separated upwards and downwards with a gap corresponding to 2H vertically during changing of head from SP head to EP head, as shown in FIG. 3. Alternatively, the gap corresponding to 2H appears inwardly reduced in the picture during changing of head from EP head to SP head. Moreover, since there is a limitation in the preciseness of the horizontal spatial dimension between 2H of the double azimuth 4-head, a horizontal skew between heads occurs near the position at which the changing of head is made from SP head to EP head or vice versa. Namely by the head switching signal (the waveform as shown in FIG. 4(e) occurs a skew to make a phase shift of 2H as shown in FIG. 4(f).
Thus, a vertical skew occurs to move the picture upwards or downwards in the vertical switching position of the head because the spatial interval of the double azimuth four heads is 2H, while a horizontal skew occurs by .alpha. because the spatial interval of the double azimuth heads is 2H.+-..alpha.. Hence, the conventional circuit does not provide a clear and stable picture during the speed change.